Implemented access to the relative path property for uploading folders

[DouglasDwyer]

I needed folder-uploading support, complete with directory structure, for a separate project. Since there does not currently appear to be a way to get directory structure, I added the ability to access webkitRelativePath from the IFileInfo interface, allowing for the support of structured folders to be uploaded using <input>. IFileInfo.WebkitRelativePath returns the relative path for any input that is marked webkitdirectory, or null if the input is not configured to upload folders.

[Tewr]

In principle ok I guess. But I'm not really happy about exposing a non standard property, especially when it has a browser prefix, as it means a future breaking change.

How about a dictionary or and indexer property filled with all the non-standard properties (passing the hasOwnProperty test) of file?

[DouglasDwyer]

While I believe the webkitRelativePath property is currently implemented across all major WASM-supporting browsers, I understand what you mean. As such, I've removed the WebkitRelativePath property in exchange for the NonStandardProperties dictionary. For any Javascript File object, it contains the properties of the File prototype not covered by IFileInfo.

[Tewr]

Great stuff! I'll merge this as soon as I can test it.

[Tewr]

This works as expected. Thank you for your contribution.